Poor contacts stop model trains on the track

Engineer comes up with a clever fix to electrical contacts to keep model trains running smoothly

Refusing to accept that poor performance was normal for Z scale model
trains, an engineer sets out to devise a better electrical contact to
eliminate stalling at slow speeds

This story does not relate a saving of multi-millions of company
profits or an earth-shaking engineering fix. Rather this is about a
simple electro-mechanical design improvement to a relatively
unimportant hobby item that still made many hobbyists happy This falls
into the category of ‘spare time at home’ engineering problem solving.

Many electrical, mechanical, civil etc engineers are, will be, or have
been into the art of diorama and model railroad construction (or
railroad modeling if you prefer). So are many non-engineers both male
and female. The hobby appeals to all with an artistic and creative
bent, and offers technical challenges and satisfaction to those who are
inclined to tackle artistic and technical challenges. In their spare
time they create accurately detailed and realistic three dimensional
sculpture and animated operational miniaturized scenes past and
present.

I’ve been an avid railroad modeler for thirty years, starting with
hand-spiked HO scale (1:87), then progressing to N scale (1:160) with
good results. Then eight years ago I decided to try Z scale (1:220)
with very disappointing results at first.

The problem? Accustomed to reliable performance from HO and N scale
locomotives, I discovered that the performance of the Z scale
locomotives left much to be desired. Smooth slow speed running without
stalling was impossible to obtain no matter how clean I polished the
rails and wheels. When I called to inquire, the locomotive manufacturer
claimed that no one had ever complained about this before. The online Z
scale community accepted this as one of the drawbacks to this scale –
the locomotives just did not make very good electrical contact to the
rails. Vendors claimed “That’s just how these things work Poor
performance is normal.”

I almost gave up at that point, then I got mad and swore that
mechanical beasties the size of my pinky were not going to get the
better of me. I took them apart and saw immediately what the problem
was.

Larger scale (N, HO, S, G) model locomotives have springy (beryllium
copper or phosphor bronze) wheel wiping contacts for electrical contact
to the wheels which do the electrical pickup from the rails. This
particular brand of Z scale locomotive did not have any electrical
contact wheelwipers. The electrical path was the haphazard contact
between the wheel axles and internal axle journals. The slightest bit
of dirt in these inherently poor contact areas caused these locomotives
to consistently stall at slow (realistic) operational speeds of under
30 scale mph.
Obviously the electrical contact to the wheels had to be improved. But
how?

I tried several things. First was to verify that my failure mode
assumption was correct. I uncoiled little railroad coupler springs to
obtain springy metallic wire, then fitted this wire into the
locomotives to press on the wheels from above and electrically bypass
the axle journal contact. Saw an immediate 1000% improvement in
performance. But this approach had a problem – if the locomotive was
accidentally driven across an isolated block where the opposing block
polarity was reversed (operator error), the thin wires burned up before
the PTC devices in the power packs would shut down from the overcurrent
short circuit.

OK, I needed a more robust wheel wiping contact that can handle a few
amps during operational goofs. So I used flat phosphor bronze contacts
from another part of the same locomotive (supplied by the locomotive
manufacturer whom by then was beginning to take an interest in my
efforts) and found that they solved the problem in both wheel contact
and overcurrent tolerance. But they were a pig to install.

So finally I did some careful measurements and did a phosphor bronze
design that could be easily fitted into the existing mechanical
structure and would bear down on the tops of the wheels to press each
wheel independently onto the rail and at the same time provide
electrical continuity. Had a few thousand of them etched from a single
sheet of phosphor bronze by a local metal shop. Cost me a bit of money
while unemployed but decided the gamble was worthwhile – I was still in
the mode of custom-built model railroads and was hoping to sell a
reliable locomotive as part of the kit and kaboodle. This design worked
amazingly well, could not believe the first tests that they actually
fit into the wheel mechanism and kept running at low speed without any
more stalling.

After verifying that the slow speed stalling problem was no longer an
issue, I wanted to find out how robust the wheel wiper design was. I
built a small tabletop railroad that I could run an upgraded locomotive
24/7. More problems…
Many times after running overnight, I would get up in the morning to
find the test locomotive off the rails, in many cases on the table, in
a couple cases on the floor. For many days this was a mystery. Then I
got lucky and finally witnessed during daylight hours the cat attacking
the moving locomotive and batting it off the rails. To a cat, a Z scale
locomotive is about the size of a mouse. All subsequent tests got moved
to the garage so feline interference was no longer an issue. After 400
hours with almost undetectable wear I decided the pressure on wheeltop
approach to wiping contacts was viable.
Wheelwipers: Simple concept, superb conductivity.

Even then I did not know what I had. At a following train show I had
some of my modified locomotives running, there was another gent
displaying an incredible Z scale empire. We admired each others’
displays, then he asked me “How do you get your locomotives to run so
slow without stalling?” I showed him my wheelwipers and he said “Can I
buy some of those?”

Since then I have been selling the wheelwipers online and installing
them for those who would rather not do the work themselves. A nice
work-at-home job.
Another upside to this is that manufacturers of Z scale locomotives are
now providing product with reliable electrical wheel contact. A big
improvement in the hobby quality.

@bhmcintosh Glen, this story made my day, especially the bit about having to remove outside biological influences from the long term test setup!

Following up, that cat has passed on, and her replacement was also fascinated with the slowly moving locomotives in for repair. I had to watch her closely, scold her when she jumped up on the workbench and got too close to the test tracks, and not test-run the locomotives when away from the bench. She liked to sit on the back of my chair and supervise my repair work.

One day she watched me take apart a locomotive, and this time I let her sniff over all the little gears, shafts, bushings etc. From this inspection she made some sort of connection in her pussycat mind and realized this was NOT a mouse, but some sort of silly human contrivance. Therefore it was far beneath a cat's dignity. She never bothered my trains again.

Hint: Start small and encourage your wife to get interested in creating the artistic scenic details. Many of the creative feminine gender who began with building dollhouses have added model rail to their hobby repertoire.
My own DW learned the artistic part while I did the trackwork. Like Jack Spratt we managed. She created some very beautiful 3D landscapes out of plaster, styrofoam, and other raw materials.
Model rail is a combination of both art and science. And is fun too.

Glen, this story made my day, especially the bit about having to remove outside biological influences from the long term test setup! :-)
It's a dangerous train of thought (sorry about that) however; now the wife's worried that I'm going to add model railroading back to my stable of hobbies that take too much time and money!

Right you are. To really appreciate the visual impact of a tiny train slowly winding it's way through mountain passes, valleys, prairies, cities, bridges etc the speed should be no more than 1 boxcar per second, and in many other cases even slower. (Unless one is modeling the TGV or similar).
Thanks to all who commented, your insights are appreciated. There are so many talented individuals in the engineering community who enjoy a hobby as an extension of their engineering careers. I do not believe there is any other career that is as conducive to play-at-home activity, we are lucky to be who we are.
Have fun.

Jack, this is another reason why home-brew electronics can beat off-the-shelf commercial offerings. Had a similar problem with another brand of N scale steam locomotive where the thin wires between tender pickups and loco motor would melt the plastic drawbar under similar conditions. The PTC current limiter device in the commercial power supply took too long to heat up, too much thermal mass. By the time the PTC shut the current off the damage had been done. Yet the supply must also be capable of sufficient current to drive multiple locomotives in parallel for long periods of time.
To get around this problem I later built my own PWM power supplies with per-cycle electronic overcurrent shutdown and hiccup mode recovery. At the beginning of each pulse a current above a preset detection threshold killed the pulse in a few microseconds and waited to repeat the cycle on the next pulse. No more locomotive damage. A sidenote was that I could include realistic momentum simulation with a long time constant and the same rise/fall ramp rates, unlike the commercial RC (resistor capacitor) exponential offerings.
Other means of current limit are automotive light bulbs used as PTC to each block of track. If a short occurs the bulbs light up, the increased resistance limits the current.
Modern DCC decoders drive the motor with a pulse frequency about 15KHz. Motor inductance limits the current in this case even if the voltage is higher than the motor spec.

Very good article, many thanks. Allow me to point out that too many model RR's (all scales) are run at too-high scale speeds - most unrealistic. Thus, good "low speed", i.e. correct speed, operation is vital. Happy New Year to all!

nobody seems to be talking about limiting the current to the motor. thats what burns out the motors. the article mentions that his first attempt caused his first "simple" wires to burn out. he was using a power supply which supplied too much current. Check the motor specs to determine the max current for the motor and then limit the power supply to about 15% below that. PWM can also be used but again with a current limit. Voltage does not burn out the motor. it is I^2R that does it.

One other comment on the poor power wiping contact issue, the DCC that DWilde1 talks about runs at a Fixed AC (18 VAC I think) on the tracks, with control signals delivered on the same circuit. Modules in each using device rectify then modulate the power, so are very tolerant of poor/varying contact resistance, so low voltage problems vs contact wiping etc are less of an issue.